Orthogonal frequency division multiplex (OFDM) signals have been proposed for various communications applications. An OFDM signal consists of a number of adjacent carriers, closely spaced in frequency, and each modulated with data. With OFDM, a data stream of .chi. bps is transmitted over N subcarriers, each carrying .chi./N bps. An advantage of OFDM technology is that the symbol rate of the data stream is decreased by a factor of N, which gives a proportional increase in delay spread tolerance. That is, delay spread or inter-symbol interference due to signal dispersion can be reduced or even eliminated by inserting a guard time interval between the transmission of subsequent symbols, thus avoiding use of an equalizer as required in single carrier systems. The guard time allows delayed copies of each symbol to die out before the succeeding symbol is received. Thus, OFDM is attractive because of its ability to overcome the adverse effects of multi-channel transmission without the need for equalization. Orthogonal Code Division Multiplex (OCDM), also known as multi-code CDMA is similar to OFDM, with a difference that instead of N subcarriers, N codes are transmitted simultaneously.
A disadvantage with the transmission of both OFDM and OCDM signals is that the maximum possible peak power is N times larger than the average transmitted power. This large peak-to-average power (PAP) ratio reduces the efficiency of the corresponding transmitter power amplifier. In order to avoid distortion of the transmitted signal, a backoff (which is the reduction in average transmitted power relative to the maximum possible output power) has to be used which is about the same as the PAP ratio of the input signal. There is a certain tradeoff between distortion and power efficiency. That is, efficiency can be improved by decreasing the backoff, allowing for more distortion. The amount of distortion that can be tolerated, however, depends on spectral regulations and required error rate.
Due to the tradeoff which exists between power efficiency and distortion, a need exists for a system which maximizes power efficiency while at the same time minimizing distortion.